# [SCI] Photoelectric Effect

The **Photoelectric Effect** is the emission of electrons from a material when struck by light — explained by Einstein (1905) by proposing that light consists of discrete quanta (photons) of energy E = hν.

## Overview

When light of sufficiently high frequency strikes a metal surface, electrons are ejected. The maximum kinetic energy of ejected electrons depends on frequency, not intensity — a result impossible to explain with classical wave theory. Einstein's photon hypothesis explained it perfectly: each electron absorbs one photon of energy hν; if hν exceeds the work function φ, the electron escapes with kinetic energy hν − φ.

This was the first direct evidence for quantisation of electromagnetic energy (Planck had quantised the emission process, not the field itself). It established the wave-particle duality of light and was the key experimental foundation for quantum mechanics.

The photovoltaic effect in semiconductors is the solid-state version: photons promote electrons across the band gap, generating a current in a p-n junction. Every solar cell operates on this principle.

## Key Figures & Recognition

- **Albert Einstein** (1879–1955): Explanation of the photoelectric effect, 1905. **Nobel Prize in Physics 1921** — awarded specifically for this discovery, not special relativity.
- **Heinrich Hertz** (1857–1894): First observed the effect (1887) in EM wave experiments.
- **Philipp Lenard** (1862–1947): Quantitative measurements. **Nobel Prize 1905** (for cathode ray work; his priority dispute with Einstein was bitter).

## Seminal Papers

- Einstein, A. ["Über einen die Erzeugung und Verwandlung des Lichtes betreffenden heuristischen Gesichtspunkt." *Ann. Phys.* 17 (1905)](https://doi.org/10.1002/andp.19053220607)

## What This Enables

- **[SCI] Quantum Mechanics** — Einstein's photon hypothesis was a co-founding step of QM alongside Planck's blackbody quantisation.
- **[TECH] Solar Cells (Photovoltaics)** — The photovoltaic effect in semiconductor p-n junctions is the solid-state realisation of the photoelectric effect.

## Discovery Character
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**Surprise level**: High — The naive expectation was that light intensity (not frequency) would determine the energy of ejected electrons. Einstein's result — that frequency alone matters, and that a minimum threshold frequency exists below which no electrons are emitted regardless of intensity — was counterintuitive and directly contradicted the classical wave theory of light.
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**Mode**: Serendipitous discovery (Hertz, 1887), theoretical revelation (Einstein, 1905). Hertz observed that UV light enhanced spark discharge while investigating EM waves — an accidental by-product of a different experiment. Einstein explained it 18 years later with the photon hypothesis he himself considered revolutionary ("very revolutionary," in his own letter to a friend). The Nobel Prize awarded to Einstein in 1921 was specifically for this — not for relativity.
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# Parents

* [SCI] Blackbody Radiation & Planck's Law
* [SCI] Blackbody Radiation & Planck's Law